This invention is directed to a method and apparatus for storing energy in a flywheel system. More particularly, the invention is directed to a flywheel system having high-temperature superconducting bearings and a hubless reluctance motor-generator comprising at least one axial gap.
An essential component of most flywheel systems is a motor-generator. The motor-generators are typically used in applications where electrical energy is converted to rotational kinetic energy and vice versa. An important feature of a flywheel system is high velocity of rotation of the flywheel itself. Unfortunately, a high velocity of rotation invariably results in large centripetal forces which can stress the many components of a flywheel system, including both the flywheel and the motor-generator.
Efforts have been made to develop flywheel systems using high temperature superconducting bearings and motor-generators. For example, flywheel systems which utilize a Halbach internal-dipole concept have been made. Such flywheel systems have motor-generators with permanent magnets that rotate around a central stator coil (e.g., "hub"). The flywheel systems which utilize the Halbach internal-dipole concept are often desirable because their internal dipole field generally produces pure torque. Unfortunately, however, such systems possess characteristics that are not very favorable. For instance, flywheel systems utilizing a Halbach internal-dipole concept have brittle permanent magnets which can fracture upon being subjected to high stresses at large rotational speeds. They also tend to have dipole fields which leak out of the cylindrical region of the permanent magnet array which can result in undesirable eddy currents, hysteresis loss in magnetic materials or both. Further, like all systems with brushless permanent magnets, the flywheel systems with the Halbach internal-dipole concept suffer from idling losses in their stator coils. These characteristics prevent efficient operation in compact flywheel system designs.
It is of increasing interest to develop a method and apparatus for storing energy in a flywheel system that does not display the above-described deficiencies. This invention, therefore, is directed to a superior flywheel system that unexpectedly stores energy substantially free of, for example, eddy currents, hysteresis and idling losses.